Charles Farr, the director of the Office for Security and Counter-Terrorism, admitted the war in Syria had become "particularly challenging" for the British government.

Speaking as the Home Office’s annual report on its counter-terrorism strategy (Contest) was published, Mr Farr warned the security services were keeping close tabs on the escalating crisis.

He admitted that a large numbers of foreign fighters, including from Britain, were active in Syria.

Mr Farr, a former MI6 officer, admitted that an assessment of between 70 to 100 people having travelled from Britain to fight in Syria “wouldn’t be far wrong”.

Some 600 terrorist attacks involving Syrian groups took place last year, including 60 suicide deaths.

He said that authorities were watching the efforts by Jabhat al-Nusra to extend its activities beyond Syria “very carefully”.

His comments came amid growing fears that some fighters could be sent back to Britain, with the sole purpose to plan and commit an act of terrorism on home soil.

With Scotland Yard increasing its efforts to investigate how fighters are being recruited and how travel is organised and money raised, officials said the number of people moving in and out of the country was constantly changing.

The Home Office warning – based on assessments by British intelligence agencies – comes as ministers debate doing more to arm and support the rebels fighting the regime of Syria’s President Bashar al-Assad.

The uprising in Syria "has involved many organisations with different political views and tactics; some are connected wit and supported by al Qaeda in Iraq,” the Home Office report says.

The Home Office warning – based on assessments by British intelligence agencies – comes as ministers debate doing more to arm and support the rebels fighting the regime of Syria’s President Bashar al-Assad.

"We're in a potentially key moment,” said Mr Farr in reference to the growing threat from Syria and, North Africa and the potential impact of withdrawing troops from Afghanistan next year.

“The threat has dispersed and diversified to the point where it might require more resources, because we're having to spread those resources across a wider geographical area."

Britain has sent a variety of equipment to anti-Assad forces, but stopped short of supplying weapons.

A European Union arms embargo currently prevents EU nations supplying lethal weapons to anyone in Syria.

Earlier this month, David Cameron suggested that Britain was willing to defy that ban and arm the rebels. The Home Office report may raise doubts about such a decision.

The report, presented to Parliament by Theresa May, the Home Secretary, also highlights the risks posed to Britain by terrorists using biological weapons, and those conducting attacks using firearms.